Unread message in Messages not being set as "read"

So, I have run into a bug where iMessage syncs my normal SMS-messages with Messages on OS X.

The problem is that the message doesn't want to get read. I click on the sender (automatic SMS from a computer notifying me of an email), the (1) disappears for a second and then returns.

The message does NOT show up unread on my iPhone or iPad,

I've tried the following: - Restarted Messages - Closed the conversation - Deleted the conversation

But no luck.

I'm going to restart my computer now, but in the meantime I hope you can come with a solution. Maybe something like "Mark all as read"?